1. Field of the Invention
The present invention relates to a lockup control device of an automatic transmission, which has a lockup clutch in a torque converter and engages a lockup clutch in a predetermined operating range.
2. Description of the Prior Art
Conventionally, a lockup control device is constructed in such a manner as to disengage a lockup clutch to relieve a shock when a shifting operation is started in the state wherein the lockup clutch is engaged at a predetermined shift position.
If, however, a controlled variable of the lockup clutch is suddenly reduced to disengage the lockup clutch when a decision is made to perform the shifting operation (that is, when a shift command is detected), the engine may blow up or a shift shock may occur.
To address this problem, Japanese Patent Application Laid-open Publication No. 5-172239, for example, discloses controlling a hydraulic pressure to the minimum value (the intermediate hydraulic pressure) within such a range as not to slip the lockup clutch when a decision is made to perform a shifting operation, and then gradually lowering the hydraulic pressure to disengage the lockup clutch to thereby prevent the shift shock.
In this device, when the engagement or disengagement of the lockup clutch is performed without changing a shift position, a controlled variable of the lockup clutch at the time when a slip revolutionary speed (=an engine speedxe2x88x92turbine revolutionary speed of a torque converter) gradually becomes zero or starts changing gradually from zero is learned for each input torque of the torque converter, and the intermediate hydraulic pressure is corrected with the learned variable in the next shifting operation.
The above prior arts, however, have a disadvantage because the slip revolutionary speed at the end of the shifting operation cannot always be uniform and thus the shift shock cannot be reduced stably.
More specifically, the hydraulic pressure is lowered at a predetermined gradient from the minimum hydraulic pressure within such a range as not to slip the lockup clutch. Therefore, the slipping state of the lockup clutch at the end of the shifting operation is changed if a shifting time varies due to the variations and the deterioration of shift clutches,. This may increase the shift shock.
Moreover, the intermediate hydraulic pressure is determined from the hydraulic pressure at which the turbine revolutionary speed is equal to the engine speed when the lockup clutch is engaged or disengaged. Actually, however, the intermediate hydraulic pressure is obtained from the hydraulic pressure that is rising (lowering when the lockup clutch is disengaged) at a predetermined rate of change. Thus, there is a difference between the actual hydraulic pressure and the detected hydraulic pressure, and this makes it difficult to correctly determine the intermediate hydraulic pressure. Even if the intermediate hydraulic pressure can be set correctly, the actual hydraulic pressure may overshoot and cause the engine to blow up.
It is therefore an object of the present invention to provide a lockup control device that is able to surely prevent the blow up of an engine and the occurrence of a shift shock during a shifting operation.
The above object can be accomplished by providing a lockup control device for an automatic transmission equipped with a lockup clutch which is controlled by a hydraulic pressure regulated with a lockup duty ratio, and which is disengaged during a shifting operation, said lockup control device comprising: a lockup duty ratio control means for setting the lockup duty ratio at a predetermined duty ratio at the time an actual shifting operation is started and controlling the lockup duty ratio in such a manner that a hydraulic pressure is lowered from a hydraulic pressure corresponding to the predetermined duty ratio at a predetermined rate of change until the shifting operation is finished; and wherein the lockup duty ratio control means learning-controls the predetermined duty ratio so that a slip revolutionary speed at the end of the shifting operation can be equal to a target slip revolutionary speed.
On the start of the actual shifting operation, the lockup duty ratio is lowered at the predetermined rate of change from the predetermined duty ratio so that the slip revolutionary speed reaches the target slip revolutionary speed at the end of the shifting operation. Thus, even if the shifting time is short or the shifting time varies according to the variation, the deterioration, etc. of shift clutches, the target slip revolutionary speed is realized stably at the end of the shifting operation and the shift shock is reduced.
In one preferred form of the present invention, the lockup duty ratio control means learning-corrects the predetermined duty ratio according to a difference between a slip revolutionary speed and a target slip revolutionary speed at the end of a prior shifting operation, thereby making the slip revolutionary speed at the end of the present shifting operation closer to the target slip revolutionary speed.
This enables the slip revolutionary speed to accurately reach the target value at the end of the shifting operation.
In another preferred form of the present invention, the lockup duty ratio control means learning-corrects the predetermined rate of change according to a difference between a slip revolutionary speed and a target revolutionary speed at the end of a prior shifting operation, thereby making the slip revolutionary speed at the end of the present shifting operation closer to the target slip revolutionary speed.
This also enables the slip revolutionary speed to accurately reach the target value at the end of the shifting operation.
And this makes the time for re-engagement of the lockup clutch after the finish of the shifting operation rather short, thereby preventing an increase of fuel consumption.
In another aspect of the invention, the lockup control device comprises: first lockup duty ratio setting and maintaining means for setting the lockup duty ratio at a first duty ratio as a lower control hydraulic pressure than a control hydraulic pressure before a determination of the shifting operation within such a range as not to slip the lockup clutch and maintaining the first duty ratio until an actual shifting operation is started; second lockup duty ratio control means for setting the lockup duty ratio at a second duty ratio at the time an actual shifting operation is started and controlling the lockup duty ratio in such a manner that a hydraulic pressure is lowered from a hydraulic pressure corresponding to the second duty ratio at a predetermined rate of change until the shifting operation is finished; and wherein the second lockup duty ratio control means learning-controls the second duty ratio so that a slip revolutionary speed at the end of the shifting operation can be equal to a target slip revolutionary speed.
According to the present invention, since the lockup duty ratio is set to the first duty ratio when the decision is made to perform the shifting operation, it is possible that the lockup clutch come into the slipping state after the start of the actual shifting operation even if a shifting time is short. Moreover, since the first duty ratio realizes the minimum hydraulic pressure within such a range as not to slip the lockup clutch, the lockup clutch is prevented from slipping until the start of the actual shifting operation.
On the start of the actual shifting operation, the lockup duty ratio is lowered at the predetermined rate of change from the second duty ratio, which is determined separately from the first duty ratio, so that the slip revolutionary speed reaches the target slip revolutionary speed at the end of the shifting operation. Thus, even if the shifting time is short or the shifting time varies according to the variation, the deterioration, etc. of shift clutches, the target slip revolutionary speed is realized stably at the end of the shifting operation and the shift shock is reduced.
In yet another preferred form of the invention, the first lockup duty ratio setting and maintaining means corrects the first duty ratio and maintains the corrected first duty ratio during a next shifting operation if it detects a slip of the lockup clutch while maintaining the first duty ratio during the present shifting operation.
This prevents the blow up of the engine and reduces the shift shock.
In yet another proffered form of the present invention, if a lockup duty ratio is not a maximum value when the decision is made to perform the shifting operation, the first duty ratio and the second duty ratio or the predetermined rate of change are prohibited from being corrected.
This prevents the blow up of the engine and the shift shock due to the incorrect learning.